While there has been a great deal of work on supporting terminal mobility at the network layer (e.g., mobile Internet Protocol (IP), mobile Asynchronous Transfer Mode (ATM), 3GPP (Third Generation Partnership Project)), the resulting point-to-point connectivity solutions in conjunction with existing Internet service models (e.g., Transmission Control Protocol/Internet Protocol (TCP/IP), Hypertext Transfer Protocol (http), search engines, web portals, etc.) do not adequately address the more fundamental issue of providing scalable and efficient information services to mobile users. Information delivery to mobile users is inherently different from conventional web services because of several factors.
One, mobile users need relevant real-time information independent of where the data reside (rather than point-to-point communication with a given network address). In many applications, communication between the mobile user and other network users will be based on “information affinity” without prior knowledge of the physical address of the communicating parties.
Two, information latency is of particular importance in the mobile computing context, since many of the envisaged applications are embedded into daily life activities. The system must be capable of delivering the desired information within seconds, in contrast to the scenario of web browsing from a wired network device, where more latency can be tolerated.
Three, user context is of particular significance in mobile applications. Factors that affect the specifics of the network service delivered include subscriber location, usage mode, other open applications, terminal capabilities, available link bandwidth and cost, etc.
Four, wireless devices will periodically change their physical point of attachment to the network. This means that the network must support the capability of dynamically re-establishing connectivity via a new access point, without losing session or application level services due to timeout mechanisms, change in service gateway, etc.
Five, mobile or wireless applications may include highly proliferated scenarios (such as sensor arrays or even home networks with many devices) in which the information model involves flexible matching of providers and consumers (rather than point-to-point connectivity). In this case, assignment of global IP addresses and the use of traditional service discovery protocols to reach each wireless device may be undesirable, or infeasible, due to scalability and complexity considerations, motivating the need for an alternative network service model.
The first generation of mobile information services are based on adaptations of familiar web technologies being accessed through a wireless packet data link such as Cellular Digital Packet Data (CDPD), General Packet Radio Service (GPRS) or 802.11b. (802.11 refers to a family of specifications developed by the IEEE for wireless local area network (LAN) technology. 802.11b is an extension to 802.11 that applies to wireless LANs and provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band.) In some cases, proprietary gateway protocols (such as Wireless Application Protocol (WAP)) have been devised as a means for adapting conventional web services to the bandwidth-limited wireless channel. See for example, the WAP 2.0 technical specifications, available from the WAP Forum at www.wapforum.org. The resulting services have received limited market acceptance due to performance or usage limitations caused by one or more of the factors mentioned above. The basic problem is that of trying to use available point-to-point network services (e.g., http, TCP/IP) to provide real-time, location-dependent information adapted to the capabilities of mobile terminals. User feedback indicates that the familiar web browsing model used in the wired Internet is not appropriate for mobile devices intended to facilitate daily life activities in real-time. Serious performance problems (such as frequent session timeouts) have been reported with centralized processing gateways and mobile service portals accessed via conventional point-to-point protocols like TCP/IP.
Multicasting-based “publish-subscribe” information service models have long been recognized as a potential solution to some of the above problems, since this approach avoids the inefficiencies of providing an inherently distributed and multipoint information retrieval solution on a point-to-point network service. IP multicast has been proposed as a means for efficient multicasting over the Internet, and represents an important step towards solving the mobile information service problem. However, while IP multicast addresses the issue of efficient multicast routing, it still leaves open the more important issue of how information is mapped onto logical multicast “channels”. There are commercial solutions which use multicasting (e.g., Tibco™ Software, Inc. 3165 Porter Drive, Palo Alto, Calif. 94304), but the critical process of mapping information onto a linear list of multicast channels remains a basic scalability bottleneck. While IP multicast solves an important part of the problem, its value is fundamentally limited by the fact that end users must have prior knowledge of the semantics associated with each multicast channel. Because of the one-dimensional nature of the multicast channels, this semantic mapping is often coarse and requires additional filtering in the receiver to reduce the information to what the application needs. This approach requires complex, application specific logic to map information to channels. It is also potentially wasteful of network and the terminal CPU resources, which is a significant issue for wireless networks and battery powered mobile devices.